Well fastest wouldn't be a bot that alches since alching is terrible exp/h.
You would want to string amulets or plank make for high exp per hour, you can do that with ArteMage from rsbuddy but I'm sure there are alternatives to this premium bot.
THIS IS A HORRIBLE HORRIBLE IDEA
You're looking at a 90% ban rate if you use an autoclicker to alch. Go to RsBuddy and use a free alcher. Also, there is a very good payed bot with a quite low ban rate.
Garys Hood is actually very great. Stand at the Soul Wars bank in the activity world. You'll get 99 mage in no time. I personally leveled 95-99 in two days.